Method of manufacturing a semiconductor device having ohmic electrode

ABSTRACT

A semiconductor device having an ohmic electrode having a satisfactory ohmic contact to an n-type GaAs can be obtained by heat treatment at low temperature. A method of manufacturing the semiconductor device having the ohmic electrode includes two processes. In the first process, a metal layer containing Ni, Sn and AuGe is formed on one main surface of the n-type GaAs. In the second process, the n-type GaAs is subjected to a heat treatment at a temperature which is equal to or higher than 190 C. and equal to or lower than 300 C. Thus, the ohmic electrode is formed on the one main surface of the n-type GaAs.

BACKGROUND OF THE INVENTION

1. Field of the Invention:

The present invention relates to a semiconductor device having an ohmicelectrode and a method of manufacturing the same.

2. Description of the Related Art:

A III-V compound semiconductor device or various kinds of II-VI compoundsemiconductor devices including a II-VI compound semiconductor lightemitting device often employ an arrangement in which a semiconductorlayer is epitaxially grown on a GaAs substrate.

This arrangement requires a process of forming an ohmic electrode on theGaAs substrate. The ohmic electrode having ohmic contact to an n-typeGaAs substrate includes an electrode made of Ni and AuGe disclosed in apaper by Sharma et al. in "SEMICONDUCTORS AND SEMIMETALS" Vol.15, p1.This thesis discloses that when the electrode is manufactured byevaporating AuGe on the GaAs substrate and further evaporating Ni on theevaporated AuGe and thereafter the manufactured electrode is subjectedto a heat treatment, during the heat treatment, a contact resistance ofthe electrode is sharply lowered around 350° C. and have a minimum valuewhen the temperature of the heat treatment is within the range of 400°C. to 450° C.

A paper by Kelly et al. in "ELECTRONICS LETTERS" Vol. 14, No. 4 (1978)discloses an ohmic electrode made of Au-SnNi-Au as the ohmic electrodehaving ohmic contact to the n-type GaAs substrate. Study of FIG. 1 inthis paper reveals that a contact resistance of the ohmic electrode canhave a minimum value only at a temperature of 300° C. or higher.

A paper by Aydinli et al. in "J.Electrochem. Soc." Vol. 128, No. 12(1981) discloses an ohmic electrode made of Au/Ni/SnNi. This paperdiscloses that when samples of the ohmic electrode are respectivelysubjected to heat treatments at 232° C., 328° C., and 420° C., nodiffusion of metal into the GaAs substrate is observed in a samplesubjected to the heat treatment at 232° C. but an ohmic contact isachieved in each of the samples subjected to the heat treatment at 328°C. and 420° C. though each of them does not have a mirror surface on itssurface.

A paper by Okuyama et al. in "ELECTRONIC LETTERS" Vol. 28, No. 19 (1992)discloses a II-VI compound semiconductor laser employing an n-type GaAssubstrate . This paper discloses that the n-type electrode is made ofIn.

A heat treatment at high temperature is necessary in order that the Inelectrode has a satisfactory ohmic contact to the n-type GaAs substrate.

Since, for example, a II-VI compound semiconductor light emitting devicecan emit short wavelength light, e.g., blue light, the II-VI compoundsemiconductor light emitting device attracts much attention as a lightsource which allows high recording density in optical recording andreproduction and allows higher resolution in photolithography. The II-VIcompound semiconductor light emitting device is formed by epitaxiallygrowing at least an n-type cladding layer, an active layer and a p-typecladding layer which compose a semiconductor light emitting device,e.g., a semiconductor laser on the n-type GaAs substrate by some propermethod such as molecular beam epitaxy (MBE) or the like. If the II-VIcompound semiconductor light emitting device is subjected to theabove-mentioned heat treatment at high temperature for forming an ohmicelectrode with a low resistance in a state that such semiconductorlayers are eptaxially grown, then it leads to generation and growth oflattice defects such as stacking fault or the like. Further, suchgeneration and growth lower a light emission characteristic and alifetime of the semiconductor light emitting device. Therefore, it isdesirable to avoid carrying out the heat treatment at high temperature.

In order to avoid such heat treatment, there can be considered a methodin which an n-type electrode is formed on the GaAs substrate before thesemiconductor layers are epitaxially grown. However, when semiconductorlayers are epitaxially grown on the gaas substrate in such electrodeforming method, impurities may be introduced into semiconductors orsemiconductor layers may be contaminated with impurities, therebysatisfactory epitaxial growth being prevented. Therefore, it isdesirable to avoid such electrode forming method as much as possible.

When the semiconductor layers are epitaxially grown on the GaAssubstrate and then a thickness of the GaAs substrate is decreased byplaning and grinding the GaAs substrate from its rear surface, it isimpossible to employ the method of forming the electrode on the rearsurface of the GaAs substrate before the epitaxial growth of thesemiconductor layers.

When the ohmic electrode is formed on the n-type GaAs substrate by aheat treatment at low temperature, it is possible to employ somemethods, e.g., a method of increasing electron density in the n-typeGaAs substrate.

However, when the electron density in the n-type GaAs substrate isincreased, defect density in the n-type GaAs substrate is increased.

SUMMARY OF THE INVENTION

In view of such aspects, an object of the present invention is toprovide a semiconductor device having an ohmic electrode having an ohmiccontact to a n-type GaAs and a method of manufacturing the same whichcan improve a light emission characteristic of a light emitting deviceformed of a II-VI compound semiconductor, for example, and increase alifetime of the light emitting device.

According to a first aspect of the present invention, a method ofmanufacturing a semiconductor device having an ohmic electrode includestwo processes. In the first process, a metal layer containing Ni, Sn andAuGe is formed on one main surface of the n-type GaAs. In the secondprocess, the n-type GaAs and the metal layer are subjected to a heattreatment at a temperature which is equal to or higher than 190° C. andequal to or lower than 300° C. Thus, an ohmic electrode is formed on theone main surface of the n-type GaAs.

According to a second aspect of the present invention, a semiconductordevice having an ohmic electrode is proposed in which an n-type ohmicelectrode made of metals including Ni, Sn and AuGe and formed on onemain surface of an n-type GaAs, and at least an n-type cladding layer,an active layer, a p-type cladding layer and a p-type ohmic electrodeare formed on the one main surface or the other main surface of then-type GaAs. At least one of the n-type cladding layer and the p-typecladding layer is formed of a II-VI compound semiconductor layer.

According to the present invention, even if the ohmic electrode isformed on the n-type GaAs after the II-VI compound semiconductor layeris formed thereon, it is possible to provide the semiconductor devicehaving the ohmic electrode having the satisfactory ohmic contact withlow contact resistance without lattice defect being increased.

The reason for this advantage is as follows. Specifically, according tothe present invention, since the ohmic electrode is formed of Sn havinga low melting point, AuGe having a low melting point and containing Geserving as an n-type impurity for the GaAs, and Ni having satisfactoryadhesion to the GaAs and having an effect to prevent cohesion of themetal layers forming the electrode, i.e., a so-called ball-up, after theheat treatment, it is possible to satisfactorily form the ohmicelectrode on the n-type GaAs at the low heat treatment temperature of300° C. or lower.

Moreover, since the metal layers of Ni, Sn and AuGe are successivelylaminated on the n-type GaAs in that order, it is possible to moreeffectively carry out diffusion of Sn and Ge into the GaAs andprevention of the ball-up or the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing a II-VI compound semiconductorlight emitting device which is an example of a semiconductor devicehaving an ohmic electrode according to the present invention.

FIG. 2 is a cross-sectional view of an ohmic electrode portion used toexplain a method of manufacturing the semiconductor device having theohmic electrode according to the embodiment of the present invention;

FIG. 3 is a perspective view of a sample used for measurement of acurrent-voltage characteristic in the ohmic electrode of thesemiconductor device having the ohmic electrode according to theembodiment of the present invention;

FIG. 4 is a graph showing the current-voltage characteristic of thesample shown in FIG. 3;

FIG. 5 is a graph showing correlation between a heat-treatmenttemperature and a contact resistance in the ohmic electrode of thesemiconductor device having the ohmic electrode according to theembodiment of the present invention; and

FIG. 6 is a cross-sectional view of the ohmic electrode portion used toexplain a method of manufacturing the semiconductor device having theohmic electrode according to another embodiment of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A semiconductor device having an ohmic contact and a method ofmanufacturing the same according to an embodiment of the presentinvention will hereinafter be described with reference to theaccompanying drawings. In this embodiment, the present invention isapplied to a II-VI compound semiconductor light emitting device shown inFIG. 1, e.g., a semiconductor laser, having semiconductor layers formedon its n-type GaAs substrate, and a method of manufacturing the II-VIcompound semiconductor light emitting device.

In this embodiment, as shown in FIG. 1, the II-VI compound semiconductorlight emitting device has an n-type GaAs substrate 1, a buffer layer 2formed on one main surface la thereof, and a II-VI compoundsemiconductor layer 3 formed on the buffer layer 2. The buffer layer 2is formed by epitaxially growing an n-type GaAs layer, an n-type ZnSelayer and an n-type ZnSSe layer on the one main surface 1a of the N-typeGaAs substrate 1 successively. The II-VI compound semiconductor layer 3is formed by successively growing a first cladding layer 31 made ofn-type ZnMgSSe, a first guide layer 32 made of ZnSSe, an active layer 33having a single quantum well (SQW) structure formed of ZnCdSe, a secondguide layer 34 made of ZnSSe, a second cladding layer 35 made of p-typeZnMgSSe, a cap layer 36 made of p-type ZnSSe, a multiple quantum well(MQW) structure layer 37 having p-type ZnSe thin films and ZnTe thinfilms which are laminated repeatedly, and a contact layer 38 made ofp-type ZnTe, on the buffer layer 2 by the MBE. When each of thesemiconductor layers of the II-VI compound semiconductor light emittingdevice is grown epitaxially, a metal organic chemical vapor deposition(MOCVD) method may be employed instead of the MBE method.

The II-VI compound semiconductor layer 3 is etched from its contactlayer 38 side to the cap layer 36 through the MQW structure layer 37 toremove its both side portions with a stripe operation portion locatedtherebetween in the direction perpendicular t o the sheet of FIG. 1being left. Insulating layers made of polyimide resin, Al₂ O or the likeare deposited in the etched portions. Thus, a current confinementportion 39 is formed.

A p-side ohmic electrode 40 is formed on the contact layer 38 bydepositing a metal layer having multilayer structure formed of Pd, Ptand Au thereon.

An n-side electrode 4 is deposited on a rear surface of the GaAssubstrate 1, i.e., on the other main surface 1b thereof which is locatedon the opposite side of the one main surface 1a on which the II-VIcompound semiconductor layer 3 is formed.

As shown in FIG. 2, the n-side electrode 4 has a Ni thin film 41 havinga thickness of 8 nm, an Sn thin film 42 having a thickness of 50 nm, andan AuGe thin film 43 (a composition ratio of Ge is 12%) having athickness of 300 nm which are successively laminated on the other mainsurface 1b of the n-type GaAs substrate 1 by a proper vacuum evaporatingapparatus.

After being laminated on the other main surface 1b of the n-type GaAssubstrate 1, the thin films 41, 42 and 43 are subjected to a heattreatment at 200° C. in hydrogen atmosphere for five minutes, therebybeing alloyed. Thus, the n-side electrode 4 which is an ohmic electrodemade of Ni, Sn and AuGe is formed.

In order to examine characteristics of the ohmic electrode composing theabove n-side electrode 4, a sample 50 shown in FIG. 3 was manufacturedas follows. Specifically, as shown in FIG. 3, the Ni thin film 41 havinga thickness of 8 nm, the Sn thin film 42 having a thickness of 50 nm,and the AuGe thin film 43 having a thickness of 300 nm were successivelylaminated on the other main surface 1b of the n-type GaAs substrate 1 bya proper vacuum evaporating apparatus, thereby the n-side electrode 4being formed. The n-side electrode 4 was subjected to the heat treatmentat 200° C. in the above hydrogen atmosphere for five minutes. Thus, thesample 50 having a pair of n-side electrodes 4 located in parallel onthe GaAs substrate 1 was manufactured. A current-voltage characteristicof the sample 50 was measured. The measured results are shown in FIG. 4.

Study of FIG. 4 reveals that a ratio of a voltage to a current, i.e., aresistance is constant and hence a satisfactory ohmic characteristic isachieved.

Further, a contact resistance of the ohmic electrode of thesemiconductor device having the ohmic electrode according to the presentinvention was measured with a temperature during the heat treatmentbeing changed. FIG. 5 shows results of the measurement. Study of FIG. 5reveals that the contact resistance was lowered to about 10⁻⁵ (Ohm cm²)when the heat treatment was carried out at 190° C. or greater and thatthe contact resistance became especially low when the heat treatment wascarried out at the temperature ranging from 200° C. to 225° C.

The temperature at which the heat treatment is carried out to form then-side electrode 4 is determined in consideration of a requiredresistivity and a degree of deterioration of the II-VI compoundsemiconductor light emitting device. Study of the temperaturecharacteristic of the contact resistance shown in FIG. 5 reveals thatthe resistivity becomes minimum when the temperature of the heattreatment is around 225° C. On the other hand, as described above, theII-VI compound semiconductor light emitting device tends to haveproduced lattice defects as the temperature of the heat treatment ishigher, particularly when it exceeds 300° C. In view of these facts, theheat treatment is carried out at the temperature ranging from 190° C. to300° C., preferably from 200° C. to 250° C.

A thickness of the formed Ni thin film 41 is preferably set within therange from 5 nm to 15 nm. When the thickness is thus set, it is possibleto lower the contact resistance of the n-side electrode 4 relative tothe GaAs substrate 1. Specifically, when the thickness of the Ni thinfilm 41 is smaller than 5 nm, it is impossible to achieve a sufficienteffect presented by forming the Ni thin film. When the thickness of theNi thin film 41 exceeds 15 nm, the Ni thin film 41 prevents diffusion ofGe, thereby lowering an effect presented by Ge as an n-type donor. Ineach of the above cases, the ohmic characteristic of the n-sideelectrode 4 is lowered.

A thickness of the AuGe thin film 43 is set within the range from 50 nmto 200 nm, e.g., to about 150 nm. When the thickness is thus set, it ispossible to prevent the AuGe thin film 43 from being peeled off from theSn thin film 42 when the heat treatment for alloying Ni, Sn and AuGe isbeing carried out.

When a ratio of Ge in an alloy composition of the above AuGe thin film43 is set to 12% , a melting point of the AuGe thin film 43 becomes thelowest temperature (i.e., 365° C.). At this time, it is possible toselect lower temperature of the heat treatment, which facilitatesalloying the AuGe thin film 43. Accordingly, it becomes facilitated toform the ohmic electrode 4.

FIG. 6 shows another embodiment in which metal thin films are furtherlaminated on the n-side alloyed electrode 4 shown in FIG. 1 and 2. WhenNi, Sn and AuGe of the n-side electrode 4 are alloyed and then a Ti thinfilm 51, a Pt thin film 52 and a Au thin film 53 are successivelylaminated on the n-side electrode 4 as shown in FIG. 6, it is possibleto obtain the ohmic electrode having an excellent adhesion. In thisarrangement shown in FIG. 6, the thicknesses of the Ti thin film 51, thePt thin film 52, and the Au thin film 53 are set to 5 nm, 10 nm, and 300nm, respectively.

The other main surface 1b of the GaAs substrate 1 on which the n-sideelectrode 4 is to be deposited may be subjected to a lapping processingbefore the n-side electrode 4 is deposited thereon. This lappingprocessing is carried out by grinding the other main surface 1b of theGaAs substrate 1 mechanically and chemically such that the GaAssubstrate 1 has a predetermined thickness, e.g., about 100 μm. Theabove-mentioned materials, for example, of Ni, Sn, and AuGe composingthe n-side electrode 4 are successively deposited on the ground mainsurface 1b by a proper method such as evaporation or the like and thenalloyed by the heat treatment. This lapping processing is carried outwhen a film thickness of the GaAs substrate 1 is decreased in order tofacilitate a fine working of the semiconductor light emitting deviceshown in FIG. 1.

While the semiconductor light emitting device, e.g., the semiconductorlaser formed of the II-VI compound semiconductor layer 3 is formed onthe n-type GaAs substrate 1 in this embodiment, the present invention isnot limited thereto. The present invention can be applied to fabricationof a semiconductor device having an ohmic electrode which is formed ofother semiconductor devices or other compound semiconductor layer or thelike.

While in the above-mentioned embodiment the semiconductor device (thesemiconductor light emitting device in the embodiment) is formed byepitaxially growing the semiconductor layer 3 on the one main surface 1aof the GaAs substrate 1 and the ohmic electrode 4 is formed on the othermain surface 1b thereof, the present invention is not limited thereto.The present invention can be applied to formation of the semiconductordevice and the electrode 4 having the ohmic contact to the GaAssubstrate 1 on the same main surface of the GaAs substrate 1. Forexample, the present invention can be applied to a semiconductor lightemitting device proposed in "a semiconductor color light emittingdevice" (Japanese laid-open patent publication No. 48286/1994) filed bythe same assignee, the semiconductor light emitting device beingarranged such that a large number of light emitting devices are formedand arranged on one main surface of a GaAs substrate and an electrode isformed on the same main surface.

According to the present invention, it is possible to form the ohmicelectrode 4 having a small contact resistance on the n-type GaAssubstrate 1 by the heat treatment at a low temperature of 300° C. orlower, e.g., of about 200° C. Therefore, it is possible to deposit theohmic electrode 4 on the n-type GaAs substrate 1 after the II-VIcompound semiconductor layer 3 is epitaxially grown thereon.Specifically, when the II-VI compound semiconductor light emittingdevice is manufactured, even if the II-VI compound semiconductor layer 3composing the II-VI compound semiconductor light emitting device isgrown on the n-type GaAs substrate 1 by epitaxy and thereafter then-side electrode 4 is formed on the n-type GaAs substrate 1, it ispossible to avoid the deterioration of the characteristics of the II-VIcompound semiconductor light emitting device, such as generation of thelattice defects or the like, and a factor to lower the lifetime thereof.As a result, it is possible to manufacture the II-VI compoundsemiconductor light emitting device, e.g., the blue laser, havingsatisfactory characteristics.

As described above, since the electrode 4 can be formed after the II-VIcompound semiconductor layer 3 is grown by epitaxy on the GaAs substrate1 and then this GaAs substrate is subjected to the lapping processing,it is possible to increase the lifetime of the semiconductor lightemitting device such as a laser or the like. Since the whole thicknessof the semiconductor light emitting device can be decreased in thiscase, it is possible to increase the degree of freedom in design of thesemiconductor device employing such light emitting device.

Having described preferred embodiments of the present invention withreference to the accompanying drawings, it is to be understood that thepresent invention is not limited to the above-mentioned embodiments andthat various changes and modifications, such as changes of thickness ofa thin film, changes of conditions of a heat treatment or the like, canbe effected therein by one skilled in the art without departing from thespirit or scope of the present invention as defined in the appendedclaims.

What is claimed is:
 1. A method of manufacturing a semiconductor device having an ohmic electrode, comprising the steps of:forming a metal layer containing Ni, Sn and AuGe on one main surface of an n-type GaAs; and subjecting said n-type GaAs and said metal layer to a heat treatment at a temperature which is equal to or higher than 190° C. and equal to or lower than 300° C., whereby an ohmic electrode is formed on said one main surface of said n-type GaAs.
 2. A method of manufacturing a semiconductor device having an ohmic electrode according to claim 1, wherein said metal layer is formed by forming Ni, Sn and AuGe successively from the side of said n-type GaAs in that order.
 3. A method of manufacturing a semiconductor device having an ohmic electrode according to claim 1, wherein before said metal layer is formed, a II-VI compound semiconductor layer is formed on said one main surface or the other main surface of the said n-type GaAs.
 4. A method of manufacturing a semiconductor device having an ohmic electrode according to claim 3, wherein said II-VI compound semiconductor layer is formed by a molecular beam epitaxy method.
 5. A method of manufacturing a semiconductor device having an ohmic electrode according to claim 1, wherein after said heat treatment is carried out, metal layers made of Ti, Pt and Au are successively formed on said metal layer made of AuGe in that order. 